Dementia is one of the greatest global health challenges we face in the 21st century. Frontotemporal dementia (FTD) is the second most common form of dementia among people under the age of 60 and its pathogenic mechanisms are poorly understood. FTD and amyotrophic lateral sclerosis (ALS), a predominantly motor neuron disease, share many clinical, pathological, and genetic features. The rapid identification of an array of genes that cause FTD/ALS has opened exciting opportunities to dissect shared pathogenic molecular pathways that may be effective targets for therapeutic intervention. In this appalication, we study the most common genetic cause of FTD/ALS, a GGGGCC repeat expansion in the C9ORF72 gene. This genetic mutation may cause disease through multiple mechanisms, including neurotoxicity induced by dipeptide (DPR) proteins generated through repeat-associated non-AUG (RAN) translation. To study these diverse pathogenic mechanisms, we take advantage of the power of Drosophila genetics including genetic suppressors and enhancers of different FTD/ALS disease genes. Such studies often reveal totally unexpected molecular pathways that shed light on pathogenic mechanisms and suggest novel therapeutic targets. In addition, cortical neurons differentiated from patient-specific induced pluripotent stem cells (iPSCs) and human patient brain tissues will be used as a complementary approach. This integrated approach?combining genetic, cellular, molecular, electrophysiological, and bioinformatics analyses?will enable us to make significant contributions to dementia research in the years to come.